Maintenance of natural grass turf on athletic playing or landscaped areas is expensive, natural grass does not grow well within shaded enclosed stadiums and continuous heavy traffic wears out areas in the natural turf surface. Natural turf surfaces deteriorate under heavy use and exposed soil creates an undesirable accumulation of water and mud. Synthetic grasses therefore have been developed in order to reduce the expenses of maintaining heavily used athletic playing areas, to render playing surfaces more uniform, and increase the durability of the grass surface, especially where professional sports are involved.
Synthetic grass is installed with a carpet-like pile fabric having a flexible backing laid on a well drained compacted substrate, such as crushed stone or other stabilized base material. The pile fabric has rows of upstanding synthetic ribbons representing grass blades extending upwardly from the top surface of the backing.
Of particular interest to the present invention are the various formulations for granular resilient fill that are placed between the upstanding ribbons on the upper surface of the backing to simulate the presence of soil. Most prior art systems involve some use of hard particles such as sand or crushed slag particles, together with resilient particles such as crumb rubber particles or foam backing to provide resilience. The optimal choice of particle sizes, particle shape, particle composition and installation in multiple layers or courses is a feature of the present invention.
U.S. Pat. No. 4,337,283 to Haas, Jr. discloses a homogeneous infill mixture to imitate soil that is made of fine hard sand particles mixed with 25% to 95% by volume resilient particles to provide an improved resilient and less abrasive infill. Such resilient granular material may include mixtures of granulated rubber particles, cork polymer beads, foam rubber particles, vermiculite, and the like.
U.S. Pat. No. 4,396,653 to Tomarin discloses a non-homogeneous infill with rubber particles forming a base layer and sand particles forming a top layer. The rubber particles provide inner resiliency to the surface. The sand layer is exposed and forms a stabilizing cover layer for the underlying rubber particle layer.
A number of disadvantages result from the use of a uniformly mixed granular infill as in the Haas system where hard sand particles and resilient rubber particles are mixed and blended in a uniform proportion throughout the depth of the infill. Synthetic grass infill, for example, may comprise a mixture of 60% by weight of sand and 40% granulated rubber particles uniformly mixed and deposited between the upstanding synthetic grass ribbons to a depth of 1 to 3 inches.
A high percentage of sand is preferred to minimize the cost of such systems, since rubber particles are relatively expensive compared to sand. The sand particles also provide an improved degree of drainage that is needed where the synthetic grass surface is not in an enclosed stadium for example. Rubber particles tend to impede the free flow of water, whereas the capillary action of the sand particles draws surface moisture downwardly due to the differences in surface tension characteristics between rubber and silica sand.
However, in both the Haas and Tomarin systems, abrasive hard sand particles present in the top surface layer of infill causes problems where such as games of football, rugby, soccer, field hockey, baseball are played since players repeatedly fall down or are knocked down on the playing surface. In such applications, there is a need to protect players from skin abrasion caused by the hard sand in the granular infill and from sand spraying into the players eyes, ears and mouth.
The conventional infill is a mixture of sand and rubber particles. The rubber particles are compressed and released when a ball hits the surface or an athlete steps on the surface. In the case of conventional soil, the soil and humus particles provide some natural resilience but the rebound is more gradual due to moisture, small particle size and relatively low natural resilience. In the case of synthetic infills, the particles are relatively dry and do not bond together. The rubber particles have a spring-like rapid resilient rebound that tends to hurl adjacent sand particles and rubber upwardly under force.
The synthetic infill is continuously subjected to water flow and impact forces that tend to dislodge or segregate the particles, such as from rainfall, flooding, the impact of bouncing balls, vibration and impact from the feet and bodies of players in contact with the top surface of the infill. A top layer with a high proportion of sand will result in spraying of sand particles when a ball or player impacts with the top surface of the infill. When soccer balls roll on the infill surface, if any sand particles are present at the top surface, sand particles are lifted by the rolling ball by the suction force of air flowing around the spinning ball and by static electric attraction. As a result the smaller sand particles on the top surface of the infill are lifted and sprayed in a “rooster tail” pattern behind the rolling ball. Over time, areas of continuous sand spray or ball impact will result in visible sand on the playing surface. It is considered undesirable to have light colored sand visible in the synthetic grass surface and, especially when clouds of sand are visible on such impacts. In addition, exposed sand granules are highly abrasive to the skin when players fall or slide on the top surface, and could irritate eyes, ears, nose and mouth when sprayed, inhaled or ingested.
A further disadvantage of conventional infills is that abrasive sand particles remain on the top surface of the synthetic grass and players on the surface who come in contact with the sand particles experience skin abrasion. Over time, due to the dynamics of water, vibration and impact, the smaller sand particles will tend to settle toward the bottom of the infill layer and larger more abrasive sand particles will rise to the top surface. The small sand particles tumble downward in the voids between larger particles under the influence of vibration, water and gravity. Smaller particles accumulate at the lower portion of a granular infill layer and tend to compact together. The larger sand particles remain at the top of the granular layer and large particles are highly abrasive to human skin relative to the smaller particles.
As a result, over time the abrasive nature of the synthetic system is increased and may result in particular areas of the playing surface which experience heavy traffic being more abrasive than other areas. Conventionally used hard particles and resilient particles have angular surfaces. It has been found however that angular particles tend to compact together more than spherical or rounded particles since the friction between sharp angular surfaces is greater. In addition, where a wide range of particle sizes is used, the smaller particles fill in the interstices between the larger particles and increase the degree of compaction.
When shredded rubber, or conventional ground rubber are used the rubber particles have irregular surfaces often with fibrous protrusions that trap air and hold water with surface tension. When the infill is rained on or flooded, the air trapped by the lightweight rubber particles causes the rubber particles to float. This is undesirable since the rubber may wash down a drain with the surface water flow, and the floating rubber separates from the heavier sand in the infill mixture thereby leading to particle segregation, sand compaction and loss of the resilience of the infill.
Where sand is used for construction purposes such as road building or in concrete mixes, it is highly desirable to have a wide range of particle sizes specifically because a mix of small and large particles will result in small particles filling of the interstices between large particles, increased inter-particle contact, superior compaction and therefore a higher load bearing capacity. Where sand or granular aggregates are used in construction applications, vibratory compactors are employed and moisture content is controlled to produce maximum soil compaction and density.
However, where sand is used as a component of a resilient infill between the interstices of synthetic grass, excessive compaction is highly undesirable. A high degree of compaction of sand and contamination of the infill by airborne dirt and dust lead to unwanted changes in the resiliency of the infill over time as a result of use which may vary considerably over the synthetic grass surface from areas of high use to areas of low use. Uniform consistent resilience, elimination of maintenance and predictable performance of the infill are the goals rather than high load bearing strength.
The conventional solution to the compaction and separation of infill particles is to periodically brush the synthetic grass. Brushing serves to break up compacted material and remix the top surface restoring the original composition of the infill mixture as much as possible. Brushing increases the cost of maintenance, exposes synthetic ribbons to significant wear, and is at best a temporary solution since eventually the conventional infill compacts again and must be brushed regularly.
The proper choice of spacing between rows of grass ribbons has also proven to be problematic. Quite often the major complaint of professional athletes is that cleats on shoes do not release consistently from densely packed, matted, tightly woven or knitted synthetic sport grass surfaces, causing knee and ankle injuries. Older artificial grass surfaces were built much like indoor carpet surfaces with very closely spaced upstanding fibers extending from a woven base with resilient underlay. These fiber surfaces were designed to remain upstanding and avoid matting when stepped upon. Therefore to achieve this result, the fibers were spaced extremely close together. However, the cleats on athletic shoes often did not release properly especially when the foot was spun on the surface, thereby resulting in knee and ankle injuries.
On the other hand, where pure sand is used as a surface, in equestrian surfaces for example, the surface is relatively unstable and sand particles displace easily. To stabilize such surfaces, U.S. Pat. No. 4,819,933 to Armond (Fibresand Limited) provides a mixture of sand with a relatively small percentage by weight of straight synthetic fibers randomly distributed and cross-linking in a loose displaceable network. The fibers serve to distribute concentrated loads, hold the sand together under the weight of horses hooves, athletic players' feet, wheeled vehicles or implements. U.S. Pat. No. 5,326,192 to Freed (Synthetic Industries, Inc.) also provides a method of improving the appearance and performance characteristics of a turf surface by working discreet bunches of synthetic fibers into the soil surface.
Granular infill combined with upstanding grass-like synthetic ribbons address the disadvantages of the above systems to a degree by providing a granular synthetic surface intermingled with the upstanding fibers extending from a fabric backing to better imitate a natural soil, embedded roots and grass. When the cleats on an athlete's shoe embed in the granular infill, the loose particles shift and displace somewhat like natural soil. At the same time the upstanding synthetic grass ribbons enmesh with the loose particles and the cleats to reduce or prevent slipping. Without the synthetic ribbons, the loose particles would be very difficult to run on much like a dry sand natural beach surface whereas a dense mat of fibers would ensnare the cleats preventing release and possibly causing personal injury.
Therefore the combined structure of upstanding ribbons and loose particulate infill must be balanced or optimized to provide a desirable playing surface. When the ribbons are densely packed together, the cleats cannot release properly, but when the ribbons are spaced too far apart, adequate traction and stability is not available. Due to the high cost of artificial grass installations, and risk of injury to highly skilled and highly paid athletes, a predictable and reproducible artificial grass performance is required.
Synthetic grass surfaces have also been constructed with infill substantially of rubber only. Rubber particles are relatively light, and shredded particles have fibrous surfaces that trap air bubbles. As a result when flooded, the rubber particles of some conventional installations have floated on the surface of water draining off the synthetic grass surface. Rubber particles drain away or are displaced resulting in areas of the synthetic grass which have depleted infill thickness. A lack of uniform infill thickness and resilience across the surface can result in injuries and liability for the owner of the athletic field.
Despite several different rubber and sand infill compositions and fiber structures in the prior art, several significant disadvantages remain as noted above.
It is an object of the present invention to provide an infill that will retain its properties throughout use without substantial segregation or compaction of the infill and with a reduced requirement for periodic brushing of the surface.
It is a further object of the invention to enhance the resilience and reduce the abrasive nature of conventional granular infills filling the interstices of the synthetic grass ribbons while enabling the cleats of athletic shoes to properly release without serious risk of injury.
It is a further object of the invention to eliminate the spraying of sand particles and undesirable visible sand on the infill surface.